A method which comprises adding a precipitant to a metal salt solution to form precipitates of a metal hydroxide has been used to prepare magnetic powder. However, when the precipitant is externary added to such a solution, the concentration of the precipitant in the solution becomes instantaneously and locally high and the resultant precipitates are likely to cause impurities uptake even if the concentration of the precipitant is low and it is added in small portions with stirring. On the contrary, use of a homogeneous precipitation method makes it possible to provide pure and compact precipitates. This homogeneous precipitation method is a method in which a precipitant is gradually produced in the solution by a chemical reaction such as hydrolysis. According to this method, the concentration of the precipitant is always maintained at an extremely low level since the produced precipitant is immediately consumed. As a result, the incorporation of impurities into the resultant precipitates and the amount of bound water are maintained at a low level because the precipitated particles slowly grow with time and precipitates of small volume is prepared.
Among the homogeneous precipitation methods, the most frequently used is an urea method in which a metal hydroxide or oxide is prepared utilizing a hydrolysis reaction of urea. The urea method is used to prepare a hydroxide or an oxide of iron.
However, it is found that the particle size of powder of such an oxide or the like prepared by the homogeneous precipitation method is large. Accordingly, an object of the present invention is to provide a method which is suitable to prepare magnetic powder consisting of a relatively small particles such as fine particles having an average particle size ranging from about 50 to 500 nm.